1. Field of the Invention
The present invention relates to a rotary electric device and, more particularly, to a permanent magnet rotor in a permanent magnet type rotary electric device.
2. Description of Related Art
Machines powered by electric motors often use permanent magnet rotor assemblies. A typical rotor assembly of this type has a plurality of permanent magnets spaced around the periphery of a rotor core and includes lamination stacks mechanically secured to resist centrifugal forces when the shaft rotates.
One of the challenges in designing high-speed permanent magnet motors is retention of the lamination stacks in a position that does not permit movement of an individual lamina relative to one another and keeps the lamination stacks in a compressed state. Failure to hold the laminations securely results in fretting or cyclic bending of individual lamina leading to eventual fatigue.
Currently, several methods are used in permanent magnet motors and alternators to secure and compress lamination stacks. One of these methods includes bonding a stack of laminations together using a dielectric bonding agent. See, for example, Fathimulla et al. U.S. Pat. No. 6,772,503 entitled “Rotor Assembly Having Bonded Lamination Stack”. However, the use of such bonding agents limits the speed and power of permanent magnet motors or alternators due to the limited strength of the bonding agents or adhesives. An alternate method for compressing a lamination stack is to clamp the lamination stack with a central tie shaft. See, for example, Wilkin, U.S. Pat. No. 6,741,010 entitled “Rotor Disc Assembly Having Rotor Rim and Alternate Magnets and Laminated Pole Pieces” and Ifrim, U.S. Pat. No. 6,703,741 entitled “Permanent Magnet Rotor Portion for Electric Machine”. In such a system, a higher clamping load results in higher lamination stiffness. However, high clamping loads also decreases the efficiency of the permanent magnet rotor.
An improved arrangement for securing the laminations and permanent magnets is disclosed in Watson, U.S. Pat. No. 6,933,645 entitled “Permanent Magnet Rotor and Magnet Cradle” which is assigned to the same assignee as this application.
A need exits for a method of compressing and securing lamination stacks in a permanent magnet rotor such that movement between individual lamina is reduced.